I think it would be awfully close, but not exactly. YOu would have to factor in the refractive index of the glass, also. THe light is travelling at an angle through the glass, that angle is set by the refractive index. The difference in the length of the travel is not the difference in the thickness of the glass, but the the difference in the hypoteneuse of a right triangle formed by the thickness of the glass as the altitude of the triangle, and the difference in exit points of the ray which form the base of the right triangle. Change the thickness of the glass, and you lengthen the hypoteneuse even more.

Thanks Alex. Without getting too complicated, I have a "parfocal" set of LRGB's that I intend on testing with a f/4 reflector. In contrast to my "ED" doublet. I'm using a mask but I still find stars are not the same size through different filters. I'm sure there may be a lot of reasons for that, but I have to start somewhere simple.

Don Goldman advertizes that his parfocal filters are parfocal for a reflector. IThey are not necesarily for a refractor, even an apo. I find that there is a difference in the positions with mine and my reflector, but that the difference is smaller than the circle of confusion, that is, less than the critical focus zone.

I'd done some tests with my Baader RGBL and Ha/SII/OIII and found I had different responses on different (refractor)scopes. My solution was to change the way I way gathering the data, going to one filter at a time instead of filter sequence. Down side is that sometimes I cannot get all color panes for an image. I think the focal variation is to do with the relative difference with refractors even if just in mm's and fractions is different enough to require refocus - most importantly with the OIII, SII and Ha seem very close.